1. Field of the Invention
The present invention relates to an image processing apparatus for reproducing a high quality of image, and more particularly to an image processing apparatus capable of sharply reproducing both a half-tone image and a line image such as a character.
2. Description of the Prior Art
A dither method has been well known as a processing method for a color image having a half-tone by digitizing it. The dither method has been used in many fields because:
(1) it can save memory capacity of a memory unit necessary for the image processing, PA0 (2) it can represent half-tones by a binary display device, and PA0 (3) hardware implementation of a processing apparatus is easy.
However, when such a dither method is used to reproduce normal characters or lines on a display, linearity is disturbed by a dither pattern at an edge of the displayed image and the image is not clear.
When the characters or the lines are reproduced on the display, it is advisable to digitize the color image signal by a comparison with a fixed threshold.
On the other hand, when the digitization of the color image signal by the fixed threshold is applied to the display of the normal color image, a tonality of the displayed image is lost.
Accordingly, in the image processing for the color image display, it is necessary to selectively use the dither method for a color image having half-tones and use the fixed threshold method for a character and/or line image.
In the display of the color image having half-tones, black is displayed by subtractive mixing of cyan (C), magenta (M) and yellow (Y), while in the display of the characters and lines it is advisable to display them by black color only from standpoints of consumption of ink or toner and misregistration. Accordingly, it is necessary to selectively use the three-color superposition display for the color image having half-tones and the single-color display for characters and lines.